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To evaluate the transmembrane movement of chloride in a preparation of cardiac muscle lacking the extracellular diffusion limitations of natural specimens, intracellular chloride concentration ( [Cl] i) and transmembrane 36Cl efflux have been determined in growth-oriented embryonic chick heart cells in tissue culture. Using the method of isotopic equilibrium, [Cl]i was 25.1 +/- 7.3 mmol x (liter cell water)- 1, comparable to the value of 24.9 +/- 5.4 mmol x (liter cell water)-1 determined by coulometric titration. Two cellular 36Cl compartments were found; one exchanged with a rate constant of 0.67 +/- 0.12 min-1 and was associated with the cardiac muscle cells; the other, attributed to the fibroblasts, exchanged with a rate constant of 0.18 +/- 0.05 min- 1. At 37 degrees C, transmembrane Cl flux of cardiac muscle under steady-state conditions was 30 pmol x cm-2 x s-1. In K-free, normal, or high-Ko solutions, the responses of the membrane potential to changes in external Cl concentration suggested that chloride conductance was low. These results indicate that Cl transport across the myocardial cell membrane is more rapid than K transport and is largely electrically silent.